The disclosures referred to herein to illustrate the background of the invention and to provide additional detail with respect to its practice are incorporated herein by reference. For convenience, the disclosures are referenced in the following text and respectively grouped in the appended bibliography.
Retroviral vectors are the most efficient tools for the stable introduction of genes into vertebrate cells. Clinical experiments have been conducted to use retrovirus vectors to cure a genetic disease in humans (adenosine deaminase (ADA) deficiency). Besides correcting inborn errors of metabolism, gene therapy is also being tested in clinical trials to cure cancer and various other diseases (Science 1992, Vol. 258, pp. 744-746).
Retroviral vectors are basically retroviral particles that contain a crippled viral genome in which all viral protein coding sequences have been replaced with the gene(s) of interest. As a result, such viruses cannot further replicate after one round of infection without the help of a helper virus. Retroviral vector particles are produced by helper cells. Such helper cells contain plasmid constructs which express all retroviral proteins necessary for particle production and replication. After the introduction (transfection) of the retroviral vector genome into such helper cells, the vector genome (an RNA genome) is encapsulated into virus particles (due to the presence of specific encapsidation sequences). Virus particles are released from the helper cell carrying a genome containing only the gene(s) of interest. After infection of a fresh target cell, the RNA genome is reverse transcribed into DNA and the DNA copy is integrated into the host genome. The integrated viral DNA is called the provirus. In the last decade, several retroviral vector systems, derived from chicken or murine retroviruses, have been developed for the expression of various genes (for reviews see Temin, 1987; Gilboa, 1990).